1. Field of the Invention
The present invention relates to a hydraulic transmission apparatus which is able to vary the capacity of a swash plate hydraulic motor connected to a swash plate hydraulic pump by means of a hydraulic passage, by varying the angle of the swash plate of the hydraulic motor.
2. Description of the Related Art
Japanese Utility Model Publication No. 42-19728, Japanese Patent Publication Nos. 61-23412 and 32-7159, Japanese Patent Unexamined Publication No. 2-190668, etc. disclose so-called divided-transmission type hydraulic transmission apparatuses which employ mechanical transmission means for partial power transmission. Each of the apparatuses disclosed in the above-mentioned Japanese Utility Model Publication No. 42-19728 and Japanese Patent Publication No. 61-23412 comprises a hydraulic pump and a hydraulic motor which is provided around the hydraulic pump. Each of the apparatuses disclosed in the above-mentioned Japanese Patent Publication No. 32-7159 and Japanese Patent Unexamined Publication No. 2-190668 comprises a hydraulic pump and a hydraulic motor which are arranged in a line so as to face in opposite directions.
The conventional devices described in Japanese Utility Model Publication No. 42-19728 and Japanese Patent Publication No. 61-23412 requires a hydraulic motor having an increased diameter, resulting in an increased moment or inertia. Therefore, the hydraulic transmission apparatus according to these disclosures is not suitable for operation in a relatively high rotational speed range. Even if a wide range of tranmission ratios is not needed, a significant reduction of the size of the hydraulic motor is difficult because the hydraulic motor surrounds the hydraulic pump.
The conventional device according to Japanese Patent Publication No. 32-7159 and Japanese Patent Unexamined Publication No. 2-190668 can reduce the size of the hydraulic motor. However, according to these disclosures, a distributor valve positioned between the hydraulic motor and the hydraulic pump is apart from a stationary member, for example, the transmission case. Therefore, the apparatus disclosed in Japanese Patent Unexamined Publication No. 2-190668 has distributor valve adjusting means provided in a stationary member surrounding the motor cylinder block, thus requiring as many distributor valves as motor plungers. The construction of the apparatus is thus complicated. Further, because the distributor valves are spool-shape valves, they are not capable of automatically controlling the gaps in the sliding surfaces, thus requiring high-precision working to substantially eliminate the gaps to prevent fluid from leaking therefrom. In the apparatus disclosed in Japanese Patent Publication 32-7159, because the stationary shaft supporting the distributor valve provided in a middle portion of the apparatus is extended from an end portion of the apparatus, the stationary supporting shaft inevitably becomes long, thus making it difficult to achieve the high torsional rigidity required for consistent timing of the distributor valve. The distributor valve of this apparatus also lacks the above-mentioned capability of automatic gap reduction and, therefore, requires high-precision working to substantially eliminate the gaps on the sliding surface.
Japanese Utility Model Publication No. 42-19728 also describes direct transmission clutch means which is provided in an end portion of the hydraulic transmission apparatus so as to directly connect the input shaft and the motor cylinder, thus enhancing the transmission efficiency. However, this device increases the axial length of the hydraulic transmission apparatus as well as the diameter thereof, thus impeding installation of the apparatus into a vehicle, i.e., it has poor vehicle mountability.